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[Neo VM Bug] Fix negative counter issue #3327

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[Neo VM Bug] Fix negative counter issue #3327

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d03e02a
ReferenceCounter fix
shargon Jun 11, 2024
f00012e
Update src/Neo.VM/ReferenceCounter.cs
shargon Jun 11, 2024
c7431a1
Fix
shargon Jun 13, 2024
838ec24
Merge branch 'master' into core-ref-counter-fix
NGDAdmin Jun 18, 2024
655e4ca
Merge branch 'master' into core-ref-counter-fix
NGDAdmin Jun 18, 2024
dbd2993
Merge branch 'master' into core-ref-counter-fix
NGDAdmin Jun 18, 2024
f539ee7
Merge branch 'master' into core-ref-counter-fix
NGDAdmin Jun 18, 2024
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2 changes: 1 addition & 1 deletion src/Neo.VM/ReferenceCounter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,12 +115,12 @@ internal int CheckZeroReferred()
tracked_items.Remove(item);
if (item is CompoundType compound)
{
references_count -= compound.SubItemsCount;
foreach (StackItem subitem in compound.SubItems)
{
if (component.Contains(subitem)) continue;
if (!NeedTrack(subitem)) continue;
subitem.ObjectReferences!.Remove(compound);
references_count--;
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this ensures that it can be properly removed, but i think there is also problem of adding reference.

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Do you have an example?

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@cschuchardt88 cschuchardt88 Jun 16, 2024 •
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I believe if it goes negative it means the item was never met from previous one StackItem. You can use the CachedComponents to see.

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I'd expect references removal to be symmetric wrt references addition. And currently it's asymmetric because there's if (!NeedTrack(subitem)) continue; a few lines above; and during references addition we always increment refcounter irrespective of NeedTrack:

neo/src/Neo.VM/ReferenceCounter.cs

Lines 49 to 50 in b1d27f0

references_count++;
if (!NeedTrack(item)) return;

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Yes, maybe we need to move the line before that if

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It fails if is symmetric, we get a negative counter

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The thing that I'm worried about is that we should find the reason why symmetric approach doesn't work. Right now I don't completely understand it.

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}
}
item.Cleanup();
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37 changes: 37 additions & 0 deletions tests/Neo.VM.Tests/UT_ReferenceCounter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,8 @@
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Neo.VM;
using Neo.VM.Types;
using System;
using Array = Neo.VM.Types.Array;

namespace Neo.Test
{
Expand Down Expand Up @@ -240,5 +242,40 @@ public void TestArrayNoPush()
Assert.AreEqual(VMState.HALT, engine.Execute());
Assert.AreEqual(array.Count, engine.ReferenceCounter.Count);
}

[TestMethod]
public void TestNegativeReferenceCounter()
{
var referenceCounter = new ReferenceCounter();
var array = new Array(referenceCounter) { OnStack = true };
referenceCounter.AddStackReference(array);
for (var i = 0; i < 100; i++)
{
var item = new Integer(i) { OnStack = true };
array.Add(item);
}

Assert.AreEqual(101, referenceCounter.Count);
referenceCounter.CheckZeroReferred();
Assert.AreEqual(101, referenceCounter.Count);
var copyArray = array.DeepCopy(true);
Assert.AreEqual(201, referenceCounter.Count);
referenceCounter.CheckZeroReferred(); // Deepcopied value will be removed from the reference counter.
Assert.AreEqual(201, referenceCounter.Count);
// If we add the deepcopied value again, the reference counter will increase by only 1.
referenceCounter.AddStackReference(copyArray);
Assert.AreEqual(202, referenceCounter.Count);
referenceCounter.RemoveStackReference(copyArray);
// Removing the deepcopied value will decrease the reference counter by 101.
referenceCounter.CheckZeroReferred();
Assert.AreEqual(201, referenceCounter.Count);

// Bellow will trigger the negative reference counter exception
referenceCounter.AddStackReference(copyArray);
Assert.AreEqual(202, referenceCounter.Count);
referenceCounter.RemoveStackReference(copyArray);
referenceCounter.CheckZeroReferred();
Assert.AreEqual(201, referenceCounter.Count);
}
}
}